Micrometer-sized Water Ice Particles for Planetary Science Experiments: Influence of Surface Structure on Collisional Properties

• Published in: The Astrophysical journal Vol. 848; no. 2; pp. 96 - 104
• Main Authors: Gärtner, S., Gundlach, B., Headen, T. F., Ratte, J., J. Oesert, Gorb, S. N., Youngs, T. G. A., Bowron, D. T., Blum, J., Fraser, H. J.
• Format: Journal Article
• Language: English
• Published: Philadelphia The American Astronomical Society 20.10.2017; IOP Publishing
• Subjects: ACCRETION DISKS; accretion, accretion disks; AGGLOMERATION; Astrophysics; ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; COLLISIONS; Experiments; ICE; Ice particles; LAYERS; Liquid nitrogen; MELTING; methods: laboratory: solid state; NEUTRON DIFFRACTION; Neutron scattering; NEUTRONS; NITROGEN; Planet formation; PLANETS; planets and satellites: formation; Protoplanetary disks; PROTOPLANETS; SATELLITES; SIMULATION; Snowline; Solid phases; SUBLIMATION; Surface boundary layer; Surface layers; Surface structure; SURFACES; Temperature; TEMPERATURE DEPENDENCE; TEMPERATURE GRADIENTS; Temperature trends; Vapor phases; Water ice
• ISSN: 0004-637X; 1538-4357
• DOI: 10.3847/1538-4357/aa8c7f

Models and observations suggest that ice-particle aggregation at and beyond the snowline dominates the earliest stages of planet formation, which therefore is subject to many laboratory studies. However, the pressure-temperature gradients in protoplanetary disks mean that the ices are constantly processed, undergoing phase changes between different solid phases and the gas phase. Open questions remain as to whether the properties of the icy particles themselves dictate collision outcomes and therefore how effectively collision experiments reproduce conditions in protoplanetary environments. Previous experiments often yielded apparently contradictory results on collision outcomes, only agreeing in a temperature dependence setting in above 210 K. By exploiting the unique capabilities of the NIMROD neutron scattering instrument, we characterized the bulk and surface structure of icy particles used in collision experiments, and studied how these structures alter as a function of temperature at a constant pressure of around 30 mbar. Our icy grains, formed under liquid nitrogen, undergo changes in the crystalline ice-phase, sublimation, sintering and surface pre-melting as they are heated from 103 to 247 K. An increase in the thickness of the diffuse surface layer from 10 to 30 ( 2.5 to 12 bilayers) proves increased molecular mobility at temperatures above 210 K. Because none of the other changes tie-in with the temperature trends in collisional outcomes, we conclude that the surface pre-melting phenomenon plays a key role in collision experiments at these temperatures. Consequently, the pressure-temperature environment, may have a larger influence on collision outcomes than previously thought.